For example:

my $sel= ...;
my $bytes= 16*1024;
my %buf;
my @ready;
while( @ready= $sel->can_read() ) {
    foreach my $fh ( @ready ) {
        for my $buf ( $buf{$fh} ) {
            $buf = ''
                if  ! defined $buf;
            my $eof= ! sysread( $fh, $buf, $bytes, length($buf) );
            while(  $buf =~ s/^(.*\n)//
                ||  $eof  &&  $buf =~ s/^(.+)$//
            ) {
                my $line = $1;
                if( $line =~ /keyword/ ) {
                    print REDUCEDLOG $line;
                }
            }
        }
    }
}
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