#!/usr/bin/env perl
use 5.010;
use warnings;
use strict;

my $var1 = "abel";
my $var2 = "baker";
my $var3 = "charlie";

while (my $line = <DATA>) {
    chomp $line;
    say "Before interpolation: $line";
    say "After interpolation:  " . perform_interpolation($line);
    say '';
}

sub perform_interpolation {
    my $text = shift;
    # now what?
}

__DATA__
I'd like to see this one: $var1.
How about \$var3? You shouldn't interpolate \$var3 (but it would be nice if you'd remove the backslash)
Try a concatenation: $var1$var2