In addition (simple example):

use strict;

my @patterns;
my $pattern = qr{^(\w+):\w+};

do{ push @patterns, $1 while /$pattern/g }while<DATA>;

print "@patterns";

__DATA__

user_one:unused
user_two:unused
user_three:unused

__END__

STDOUT: user_one user_two user_three
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