If you only want to look up exact matches, you may want to stick the lookups in a hash rather than using substitutions in a subroutine (you might use your subroutine to access the hash instead). Maybe something like this?

my %interpret = (
    '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 $result = "43 P"; # comes from somewhere else
my $interpretation = $interpret{$result};
print $interpretation;
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-- Mike

--
just,my${.02}