[0] Perl> sub test{ print "'$_[0]' ",  ref \$_[ 0] };;

[0] Perl> test( '|' );;
'|'  SCALAR

[0] Perl> test( m/|/ );;
Use of uninitialized value in pattern match (m//) at (eval 9) line 1
'1'  SCALAR

[0] Perl> test( /|/ );;
Use of uninitialized value in pattern match (m//) at (eval 10) line 1
'1'  SCALAR