Something like this should work:

#!/usr/bin/perl 
use strict;
use warnings;
print STDERR "Enter filename: ";
my $fileName = <STDIN>;
chomp $fileName;
while (!-e $fileName)
     { 
          print STDERR "\n$fileName not found.\nRe-enter filename, or 
+q to quit: ";
          $fileName = <STDIN>;
          chomp $fileName;
          exit() if $fileName eq "q";
     }

print STDERR "$fileName exists!\n" if -e $fileName; # <-- true if the 
+file exists
[download]

-Michael

UPDATE: it'd probably be worth your while putting in a quit option too!