What you need is a "template". For a simple one, you can use String::Interpolate, but there are many more, some even implementing loops and template inheritance.

#! /usr/bin/perl
use warnings;
use strict;

use String::Interpolate;

my $template = 'String::Interpolate'->new;
$template->('http://something.com/api_stuff/$GUID');

make_call( '1234' );

sub make_call {
    my $GUID = shift;
    $template->{GUID} = $GUID;
    POST("$template");
}

use Test::More;
sub POST {
    is shift, 'http://something.com/api_stuff/1234', 'intepolates';
}

done_testing();
[download]

The same using Template:

#! /usr/bin/perl
use warnings;
use strict;

use Template;

my $template = 'http://something.com/api_stuff/[%GUID%]';

make_call( '1234' );

sub make_call {
    my $GUID = shift;
    my $template_object = 'Template'->new;
    $template_object->process(\$template, {GUID => $GUID}, \ my $outpu
+t);
    POST($output);
}

use Test::More;
sub POST {
    is shift, 'http://something.com/api_stuff/1234', 'intepolates';
}

done_testing();
[download]