You can use chop to remove a character from the end of a string, it works on $_ by default. I assume that you don't want to get rid of the line terminator so I remove it first using chomp then it gets replaced by say when the line is printed.

johngg@abouriou:~/perl/Monks$ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<__EOD__ or die $!;
this line does not match
this SUM is wrong
__EOD__

say q{};
while ( <$inFH> )
{
    chomp;
    do {
       say;
       next;
       } unless m{SUM};

    chop; chop; say;
}

close $inFH or die $!;'

this line does not match
this SUM is wro
[download]

I hope this is helpful.