I would just expand your table of city->iata examples.

my %IATA = { };
$IATA{$_} = 'LON' for qw/lon london londonuk londonengland/;
...

my $city = 'London, UK';
my $city =~ s/\W//g;
my $code = $iata{lc($city)};
print $code, $/;
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You could subsequently do a Levenshtein distance from each key, if it's not found with this scheme, to try to guess one of the most likely choices. This is the kind of thing Google does when it doesn't find many hits after you search for "Londun UK".

In general, soundex is deprecated. It's latin-alphabet-only, english-pronunciation-only, and it is not good with long words.