Don't slurp (well, 'slurp' into an array, not a scalar):

open(FILE, "$filename") or die "Cant open $filename\n";
my @lines = <FILE>;
close(FILE);

my @patternMatchArray = grep { /$myPattern/ } @lines;
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If you don't need all the lines, only the ones that match, you could do it in one shot with:

open(FILE, "$filename") or die "Cant open $filename\n";
my @patternMatchArray = grep { /$myPattern/ } <FILE>;
close(FILE);
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