/PAT/ (m/PAT/) is the match operator. qr/PAT/ compiles the pattern and returns the pattern without doing any matching. perlop 
$ perl -le'$_="abc"; print /(.)/;'
a

$ perl -le'$_="abc"; print qr/(.)/;'
(?-xism:(.))

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In reply to Re^5: Why do I need parentheses here? by ikegami
in thread Why do I need parentheses here? by rovf

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