Well, if the variable is on a line by itself then you can just create a hash of variable names with the values of what you want to interpolate. i.e.

my %hash = (
    '$example' => 'This is an example ',
    '$comment' => 'This is a comment'
);

open (FILE, 'file') or die ("Can't open file: $!\n");

while (<FILE>) {
    chomp;
    if (exists $hash{$_}) {
        print $hash{$_};
    } else {
        print $_;
    }
}

close (FILE);
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If you plan on putting the variables anywhere in the file, not just on a line by themselves, you just need to to add a foreach loop for the keys of the hash.

my %hash = (
    '$example' => 'This is an example ',
    '$comment' => 'This is a comment'
);

open (FILE, 'file') or die ("Can't open file: $!\n");

while (<FILE>) {
    chomp;
    foreach $key (keys %hash) {
        s/$key/$hash{$key}/g;
    }
    print $_;
}

close (FILE);
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But this only works if you dont have keys in the values i.e.

my %hash = (
    '$example' => '$comment is a variable',
    '$comment' => 'this is a comment'
);
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so don't do that :)