Check perlre's "Capture buffers". Also Use strict warnings and diagnostics or die!

use strict;
use warnings;

my $a = 'a.txt';
open(my $in ,'<', $a) || die "cannot open $a for reading: $!";
my $b = 'time.txt';
open (my $out,">>$b") || die "cannot create $b: $!";
while (<$in>) {
    # read a line from file $a into $_ 
    #print $out $_;
    if (/(returns \d{1} \(in \d{1}.\d{6} secs)/) {
        ## use capturing
        print $out $1."\n" ;
    }
    
} 
close($in)    || die "can't close  $a:$!"; 
close($out) || die "can't close  $b:$!";
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