You are trying to pass a whole substitution expression, not just a regex. You need eval STRING to do what you want, eval "\$nodeName =~ $regEx"; Note the escaped sigil in $nodeName. That is to prevent the variable from having its value interpolated before evaluation.

I'd be inclined to pass the regex and substitution as two arguments and interpolate them

sub changeNodeNames {
    my ($self,$node,$regEx,$subst) = @_;
    my $nodeName = $node->nodeName;
    $nodeName =~ s/$regEx/$subst/;
    # . . .
}
$foo->changeNodeNames($anode, qr/foo/, 'bar');
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