perl -MO=Deparse 
if ( '_$x_' =~ '$x') { print "match 1" }                      
if ( '_$x_' =~ /\$x/){ print "match 2" }
__DATA__
if ('_$x_' =~ /\$x/) {
    print 'match 1';
}
if ('_$x_' =~ /\$x/) {
    print 'match 2';
}
__DATA__
- syntax OK