Disclaimer: Not tested.

Maybe something like this:

my $getPatterns;
while (<>) {
    if (/^#ERRORS/ .. /^CELLS/) {
        push(@errors, $_);
        $getPatterns = 1;
        next;
    } elsif (defined $getPatterns) {
        push(@errors, $_);
        $getPatterns = undef if (/^\s*$/);
        next;
    }
    ...; # other processing
}
print "Errors:\n";
for (@errors) {
    print "    $_";
}
[download]

Update: I think this could be written as:

Update 2; No, it won't work.

while (<>) {
    if (/^#ERRORS/ .. /^CELLS/) {
        push(@errors, $_);
    } elsif (/^CELLS/ .. /^\s*$/) {
        push(@errors, $_);
    } else {
        ...; # other processing
    }
}
print "Errors:\n";
for (@errors) {
    print "    $_";
}
[download]