Your string looks like " cat mat rat pat\n", you wanna match the word 'cat' and not 'cater' or 'cats' or 'Catrina' for example, so your regex has to account for matching the word cat bounded by spaces or non word characters.. So you have many likely regex constructions to pick from (not including the look-ahead or look-behind assertions or the backtracking or the backreferences):
in reply to regex start of word (\<)
If you don't mind the letter-case of the word then you can use the i switch as in <c>m/\bcat\b/i<c>, read perlretut for a tutorial on regular expressions..
$string =~ /\bcat\b/
$string =~ /\Wcat\W/
$string =~ /\scat\s/
Update: another interesting read is 7 Stages of Regex Users...
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