#!/usr/bin/perl -w
use strict;

my $path = '/Users/johnsmith/data';
my $fname;

{
    print "Please enter a filename: ";
    chomp($fname = <STDIN>);
    last if -f "$path/$fname";
    print "File does not exist!\n";
    redo;
}
print "I have found '$fname' in '$path'.\n";