use strict;
my $output = "here is some output\n";  # for testing.. your output can come from whereever

my $out;
while ( $out !~ m/^[sSfF]$/ ) {
    print("\nInvalid input.\n Please type either S or F.\n")
        if $out;
    print("Do you want to output to (S)creen or to (F)ile?\n");
    chomp( $out = <STDIN> );
}

if ( $out =~ m/^[sS]$/ ) {
    print $output;

} elsif ( $out =~ m/^[fF]$/ ) {

    my $save;   # this is outside the FILENAME:{} block because it is needed at the end
    FILENAME: {
        while ( $save !~ (m/^[a-zA-Z][a-zA-Z_0-9]*$/) ) {
            print("\nInvalid input.\n Please begin with a letter and do not add extension, it will be added automatically.\n")
                if $save;
            print("Please enter filename:\n");
            chomp( $save = <STDIN> );
        }

        if ( -e $save ) {
            my $overwrite;
            while ( $overwrite !~ (m/^[nNyY]$/) ) {
                print("\nInvalid input.\n Please type either Y or N.\n")
                    if $overwrite;
                print("\nFilename exists. Overwrite? (Y) or (N)\n");
                chomp( $overwrite = <STDIN> );
            }
            if ( $overwrite =~ (m/^[nN]$/) ) {
                undef $save;    # reset $save to trigger new prompt!
                redo FILENAME;
            }
        }
    }

    open( DATA, ">$save" ) || die "Couldn't open $save for writing: $!\n";
    print DATA $output;
    close(DATA);

}