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use strict;
my (@keywords, %keyword)=qw/foo bar 12345 abcd/;
my ($string, %result) = "foobarfoo1234523423412345abcdefsadfabc";

$result{$_} = () = $string =~ $keyword{$_}
    for (keys %keyword);

my @regexen = map {
    qr/(?:\Q$_\E(?{$result{$_}++}))/
} @keywords;
...
    local $" = '|';
    qr/@regexen/;
};

$string =~ /$re/g;
print "$_: $result{$_}\n" for @keywords;