If your codes are that exact, do a hash lookup instead:

sub interpret_clock {
    my %codes = (
        '43 F' => 'Tunnel 1/2 Hr FT',
        '43 P' => 'Tunnel 1/2 Hr PT',
        '45 F' => 'Tunnel No Lunch FT',
        '45 P' => 'Tunnel No Lunch PT',
        'F'    => 'Full Time No Clock',
        'O'    => 'On Call No Clock',
        'P'    => 'Part Time No Clock',
        'T'    => 'Temporary No Clock',
    );
    my $code = shift;
    unless (exists $codes{$code} ) {
        die "Unknown code: ($code)";
    }
    return $codes{$code};
}
[download]

You're much less likely to be in error with that.

Cheers,
Ovid

New address of my CGI Course.
Silence is Evil (feel free to copy and distribute widely - note copyright text)