use vs require in perl

Use  and require both are used to load the modules with slight differences..

Use:
1)The loading the modules takes place at the compile time.
20The use can be used only to load the module.i.e. the .pm file.
3)the file Extensions should not be specified.
It will be used as
use sample 
if the module name is sample.pm

Require:
1)In case of require the module loading takes place at the run time.
2)The require can be used to load both module and .pl file.
3)The file Extensions has to be specified.
It will be used as
use sample.pm 
if the module name is sample.pm.

use export in perl

Exporter is generally used to import the variables and functions of the modules into the user's namespace.Here there is no need to declare the object and call the function of the modules using that object.

These are the things which have to be included in the module from where you need to export:
use Exporter.
@ISA=qw(Exporter);
@EXPORT = qw(add subtract);
@EXPORT_OK=qw(multiply divide);

Here use Exporter loads the module into the user namespace at the compile time.
@ISA is similar to the inheritance.
Here we add Exporter to the @ISA array.
@EXPORT contains the list of symbols which are exported into the user's namespace.
@EXPORT_OK contains the list of symbols which can be exported if requested by the user.
How really the export takes place:
For example:
consider the module example.pm:

use Example;
use Exporter.

@ISA=qw(Exporter);

@EXPORT = qw(add subtract);

@EXPORT_OK=qw(multiply divide);

sub add
{
($no1,$no2)=@_;
$no3=$no1+$no2;
return $no3;
}
sub subtract

{

($no1,$no2)=@_;

$no3=$no1-$no2;

return $no3;

}

sub multiply

{

($no1,$no2)=@_;

$no3=$no1*$no2;

return $no3;

}

sub divide

{

($no1,$no2)=@_;

$no3=$no1/$no2;

return $no3;

}

1;# this important to signify the end of the package.

Let us use  this module in another file sample.pl:

Content of sample.pl to use Export Module:

# in this case only the default subroutines add and subtract will be added to the user's namespace

use Example;

#But when you need multiply and divide subroutines you need to specify as these are available only on #demand basis in @EXPORT_OK.

use Example qw(multiply divide);

# once it is imported into the namespace you can directly use it

$ss=add(10,50);
print "addition results are:$ss";
#In similar way all the subroutines can be called

The significance of @ISA here is :when the perl program or module cannot find the perl program in the current module ,it will search for in the modules added in the @ISA array before throwing any error.

How this works:
When the sample.pl loads the module Example.pm  the import module in the Example.pm is called automatically making  the functions and variables to be added into the user's namespace.