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my $op = shift @ARGV;
# first element of @ARGV now in $op and removed from @ARGV.
...
print multiply(@ARGV) . "\n"  if $op eq 'multiply';
# subs as before

my $op = shift @ARGV;
# first element of ARGV in $op and removed from @ARGV.
...
print $h{$op}->(@ARGV) . "\n";

# subs as before