You can also get away with one array but only storing the non-matching lines, and printing out (or processing) the matching ones immediately. (slight rework of Plaid's code...)

my @other_lines;

while(<FILE>) {
   # whatever regex to strip out lines
               
   (/blahblah$/) ? print OUTFILE : push @other_lines, $_;
}  

print OUTFILE @other_lines;
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This way, not only do you only use one array, but you don't even store All of the lines in it- it only stores the nonmatching lines.

Alan