my @mon_names = qw/Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec/;

my %mnth_str2digit = map { $mon_names[$_] => $_ } (0..$#mon_names);

# or, if map is "unintelligible to rookies", use a for loop:

my %mnth_str2dgt;
for ( 0..$#mon_names ) {
    $mnth_str2digit{$mon_names[$_]} = $_;
]