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 $nam
+e, 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;
[download]

C:\>perl Test.pl
INTRUDER ALERT: attempting access to 'ARGV' at test.pl line 13.
Final text:
Hi I'm Joe and happen to be 42.  You can call me Joe, if you tell me y
+our $ARGV.

C:\>_
[download]