Checking if the eval succeeded finds the cause I suspected:

use strict;
use warnings;

my $op = shift;

for (qw(your hand is too cold to hold)) {
    eval $op;
    print $@ if $@;
    print $_, "\n";
}

__END__
Modification of a read-only value attempted at (eval 1) line 1.
your
Modification of a read-only value attempted at (eval 2) line 1.
hand
Modification of a read-only value attempted at (eval 3) line 1.
is
Modification of a read-only value attempted at (eval 4) line 1.
too
Modification of a read-only value attempted at (eval 5) line 1.
cold
Modification of a read-only value attempted at (eval 6) line 1.
to
Modification of a read-only value attempted at (eval 7) line 1.
hold
[download]

Fix:

use strict;
use warnings;

my $op = shift;

for (qw(your hand is too cold to hold)) {
    local $_ = $_;
    eval $op;
    print $_, "\n";
}

__END__
your
hand
is
too
cnew
to
hnew
[download]