open my $fh, '+<', 'test.txt' or die $!;

my $last_matched = 0;
my $told = 0;

while( my $line = <$fh> ) {
  my $matched = $line =~ m/SIL$/;
  truncate $fh, $told and last if $matched && $last_matched;
  $last_matched = $matched;
  $told = tell $fh;
}

close $fh or die $!;