Given the following script:

use strict;
use warnings;
1;
[download]

Commenting out the pragmas, it seems to run in about 0.004 seconds on this machine. Commenting out just use warnings, about 0.008 seconds. With both pragmas enabled, about 0.024 seconds.

This "slow down" seems pretty imperceptible unless you're running the script several times per second. (And in that case, a better speed boost could be had by just running it once, as a persistent process.)

For comparison, a similar script in Ruby takes around 0.080 seconds; in Python, around 0.050 seconds; in PHP, around 0.040 seconds; in bash, around 0.008 seconds; and in tcsh, around 0.020 seconds.