To expand on Khen1950fx's correct observation, some vars are customarily or (in this case) inherently allowed in specific cases only.

"Customarily" applies to the likes of $a and $b -- are allowed elsewhere, but, by convention, are reserved for use in sorting. OTOH, $1 is reserved for use in regex capture.

perl -e "use strict; my $1='foo'; if ($foo=~/(f)/){print $1;}"
Can't use global $1 in "my" at -e line 1, near "my $1"
Global symbol "$foo" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.
[download]

Note the problem with $foo above... despite which the attempted illegal use of $1 had precedence with the compiler's error checking.

And just to beat this deceased equine a bit more, eliminating the $foo issue (and use strict;) leads to this:

perl -e "my $1='foo'; if ($1=~/(f)/){print $1;}"
Can't use global $1 in "my" at -e line 1, near "my $1"
Execution of -e aborted due to compilation errors.
[download]

In short, use meaningful variable names... and learn which (tempting) low-keystroke vars are meaningful to Perl.

Updated penultimate para to acknowledge/highlight the elimination of strict