Why not turn it around just a little and use a regex to substitute replacement text for key text like this:

use strict;
use warnings;

my @plates = (
    {GIVENNAME => 'Fred', SIRNAME => 'Fredson', AMOUNT => '$10.24'},
    {GIVENNAME => 'Joe', SIRNAME => 'Joeson', AMOUNT => '$20.48'},
    );

my $text = do {local $/; <DATA>};

for my $plate (@plates) {
    my $copy = $text;
    
    $copy =~ s/$_/$plate->{$_}/g for keys %$plate;
    print "$copy\n\n";
}

__DATA__
Dear GIVENNAME SIRNAME,

thanks for your recent contribution if AMOUNT to PerlMonks.
[download]

Prints:

Dear Fred Fredson,

thanks for your recent contribution if $10.24 to PerlMonks.


Dear Joe Joeson,

thanks for your recent contribution if $20.48 to PerlMonks.
[download]