use strict;
use warnings;

my %legalReplacements = (name=>'Joe', age=>42);
my $text = q(Hi I'm $name and happen to be $age.  You can call me $name, if you tell me your $ARGV.);

my $regex = qr/\$([a-zA-Z]+)/;

while ($text =~ /$regex/)
{
    if (not exists $legalReplacements{$1})
    {
        warn "INTRUDER ALERT: attempting access to '$1'";
       last;
    }
    $text =~ s/$regex/$legalReplacements{$1}/;
}
print "Final text:\n";
print $text;