...
open OUTPUT, $outputFile
	or die "Can't open $outputFile for writing $!\n";
open INPUTA, $inputFileA
	or die "Can't open $inputFileA for writing $!\n";
open INPUTB, $inputFileB
	or die "Can't open $inputFileB for writing $!\n";
while($fha=<INPUTA>){
    print "a: ",$fha;
    if($fha=~m/(\d{2}):(\d{2}):(\d{2}).+(\d{2})(\d{2})(\d{2})\d{2}_\d{2}\.abc/){
	$timea=timegm($3,$2,$1,$4,$5,$6);
	$output_line=$fha;
	until($timeb>$timea){
	    $fhb=<INPUTB>;
	    print "b: ",$fhb;
	    if($fhb=~m/(\d{2}):(\d{2}):(\d{2}).+(\d{2})(\d{2})(\d{2})\d{2}_\d{2}\.abc/){
		$timeb=timegm($3,$2,$1,$4,$5,$6);
		if($timeb=$timea){
		    $output_line.=$fhb;
		}
	    }
	}
	print OUTPUT $output_line,"\n";
    }
}
close OUTPUT;
close INPUTA;
close INPUTB;