$a is not necessarily a good choice for a lexical variable's name

Actually, to be completely pendantic, $a and $b are perfectly safe so long as you use an inline sort routine in the same scope. For example, this won't work:

xyz(1 .. 5);

sub xyz {
    my $a = shift;

    sort { $a <=> $b } @_;

    print "$a\n";
}
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But, the following will work:

xyz(1 .. 5);

sub xyz {
    my $a = shift;

    abc(@_);

    print "$a\n";
}

sub abc { sort { $a <=> $b } @_; }
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Since abc() provides a different lexical scope than xyz(), everything is ok and $a within xyz() is safe.

But, your point of confusion is still good, as single-letter variables are poor.