#! perl
use strict;
use warnings;

while (my $line = <DATA>)
{
    chomp $line;
    my @recs = split /\s+/, $line;

    if ($recs[2] eq 'C4:46:19:75:C1:55')
    {
        print "ID = ", $recs[0], "\n";
    }
}

__DATA__
SS-ID VLAN MAC TIME IP RSSI MODE UAPSD BW GI WMOS DHCP IDENTITY
----- ---- --- ---- -- ---- ---- ----- -- -- ---- ---- --------
1-1 0 C4:46:19:75:C1:55 23m 192.168.5.253 -24 bgn no 20 S 4.9 ack*
1-2 0 5C:57:C8:69:8C:1E 3s 192.168.5.254 -38 bg no 20 L 4.8 ack*