I am not sure, something like this?

use strict;
use warnings;
my $file = 'match alphabets 21211 not numbers';
my $regexp = qr{[a-z]+};
$, ="\n";
print (my (@arr) = $file =~ /($regexp)/g );
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Also avoid using, undef $/, for reading file like this, which ll affect your coding in some situations, instead try Perl Idioms Explained - my $string = do { local $/; <FILEHANDLE> };.