If $DELIMITER was static and was being tested for more than once in the code I might consider making a compiled regex.

my $rxDELIMITER = qr{\Q$DELIMITER\E};
...
if ( $line =~ $rxDELIMITER ) { ...
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I probably reach for regexen too quickly without even considering the use of index. I suspect I'm not the only one with a bit of a blind spot there.

Cheers,

JohnGG