the only difference is that variable being the defaulted $_ while the other is user-defined

Nope, no "user-defined" variables being used by s/// in either snippet. 

my $r = "Time to work";
s/work/play/;
print $r;

[download]

means 

my $r = "Time to work";
$_ =~ s/work/play/;
print $r;

[download]

Since you never assigned anything to $_, you get a warning. Conversely, you never told s/// to work on $r. Doing either will solve the problem. 

my $_ = "Time to work";
s/work/play/;
print $_;

[download]
my $r = "Time to work";
$r =~ s/work/play/;
print $r;

[download]

In reply to Re^5: using s/ to not only substitute but also modify text by ikegami
in thread using s/ to not only substitute but also modify text by Anonymous Monk

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