You may have replaced $l (el) with $1 (one). $1 (one) is a reserved variable for Perl that catches parts of regular expression, and can't be assigned to directly. For example:

~/test$ perl -e '$1 = "blah"'
Modification of a read-only value attempted at -e line 1.
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Whereas :

~/test$ perl -le '$var = "blah"; $var =~ /^(....)$/; print $1'
blah
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