• You didn't turn on warnings. (You'd get a lot of them if they were on.)
• You didn't turn on strict, or your declared your variables too far from where they should be declared.
• You are printing "Invalid input" whether the input is good or bad.
• Using {1} in regexps is useless.
• You shouldn't be using regexps for constant strings.
• Why use the 2-arg open when you can use the 3-arg one? At least your file name has been validated (although possibly too strictly).
• Your file handle isn't localized. It would be even better if it was a lexical variable.
• You say an extention will be added, but you it isn't.
• You're printing too many newlines.
• You accept more file names than the error message indicates. This is not necessarily wrong.

Fix:

#!/usr/bin/perl

use strict;
use warnings;

my $output = "data";

my $out;
for (;;) {
    print("Do you want to output to (S)creen or to (F)ile? ");
    chomp( $out = uc( <STDIN> ) );
    last if $out eq 'S' or $out eq 'F';
    print("Invalid input. Please type either S or F.\n");
}

if ($out eq 'S') {
    print $output;
}
else  # $out eq 'F'
{
    for (;;) {
        print("Please enter filename: ");
        chomp( my $save = <STDIN> );

        if ( $save !~ /^[a-zA-Z][a-zA-Z_0-9]*\Z/ ) {
            print("Invalid input. Please begin with a letter and do no
+t add extension, it will be added automatically.\n");
            redo;
        }

        $save .= ".ext";  # XXX TODO

        if ( -e $save ) {
            my $overwrite;
            for (;;) {
                print("Filename exists. Overwrite? (Y) or (N) ");
                chomp( $overwrite = uc( <STDIN> ) );
                last if $overwrite eq 'Y' or $overwrite eq 'N';
                print("Invalid input. Please type either Y or N.\n")
            }

            redo if $overwrite eq 'N';
        }

        open( my $fh_out, '>', $save )
           or die "Unable to create $save: $!\n";

        print $fh_out $output;

        last;
    }
}
[download]

Update: Added the the last last. I knew I needed it!

Update: Fixed the issues raised in replies to this node.

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