You're thinking of the #line directive, which is documented in perlsyn, which reads, in part:

Much like the C preprocessor, Perl can process line directives. Using this, one can control Perl's idea of filenames and line numbers in error or warning messages (especially for strings that are processed with eval()). The syntax for this mechanism is the same as for most C preprocessors: it matches the regular expression /^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/ with $1 being the line number for the next line, and $2 being the optional filename (specified within quotes).