Some clear solutions:

my $s = join('', @_); $s =~ s/^/$prefix/mg; print $s;
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print map { $prefix . $_ } split /^/m, join('', @_);
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print apply { s/^/$prefix/mg } join('', @_);
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print join('', @_) =~ s/^/$prefix/rmg;
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apply comes from List::MoreUtils

The last requires Perl 5.14 for /r.