The way to avoid that problem when you know you are dealing with large scalars, (arrays and hashes too), is to use pass by reference. Either:

sub frobnicate {
    $_[0] =~ s/foo/bar/g;
    return;
}
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Or:

sub frobnicate {
    my ($ref) = @_;
    $$ref =~ s/foo/bar/g;
    return;
}
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Either way you avoid a lot of copying and the lingering redundant memory usage.