In general, using a foreach loop combined with a push will be clearer, however, you can do this fairly simply using map and the Conditional Operator (? :)

#!/usr/bin/perl
use strict;
use warnings;

my @ms = (
          {a => 'x1', b => 'y', c => 'z1'},
          {a => 'x2', b => 'y1', c => 'z2'},
          {a => 'x3', b => 'y', c => 'z3'},
          {a => 'x4', b => 'y2', c => 'z4'},
          );

my @list = map {$_->{b} eq 'y' ? $_->{a} : () } @ms;
print join "\n", @list;
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