Here's a variant on jwkrahn's approach that's closer to what seems to be the real goal of the OP code:

my $ps_output = $ssh->exec( 'ps efax 2>/dev/null' );

my @stat = ( $ps_output =~ /firstbo.*/g );

my $stat = scalar @stat;
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Counting matches locally is just easier. Note that the regex match here uses "firstbo.*" to make sure it matches "firstbo" only once per line (just like the remote "grep" command would have done).