In general, It is better to have a program that is easy to maintain than it is to have one that is marginally faster. ('strict' and 'my' offer several advantages to the maintainer.) The usual recommendation is to not even consider execution speed until you have a complete program that works correctly, but is unacceptably slow. At that point, profile it to identify the problem area. Write several versions of this code. Benchmark each with realistic data. (You will almost always be surprised by the result). Resist the temptation to make "easy changes" to speed-up other sections. Even a big improvement will make little difference in your complete program.

Perl's my lexical variables are not slow. They've been very heavily used in millions of lines of Perl code for many years now.

Instead of fretting about their performance, I suggest you spend your time understanding why they should be used in just about every Perl script you write. To get you started down that path, I've provided a few references below. After understanding the why, please feel free to post your code (using strict and my) for review.

If you have any performance concerns in any of your code, please present specific benchmark results first.

Why Using Strict and My Lexical Variables is Recommended

Stick to using only lexical variables (my) unless you genuinely need the functionality that only a package or punctuation variable can provide.

Using non-lexical variables increases the "coupling" of your code. If two otherwise unrelated sections of code both use a package variable, those two pieces of code can interact with each other in very subtle ways, just by the way they each interact with that shared variable. In other words, without full knowledge of every other piece of code that is called from a particular statement, it is impossible to know whether the value of a given non-lexical variable will somehow be changed by executing that statement.

-- Damian Conway in Perl Best Practices (Chapter 5: Variables)

• Information Hiding (wikipedia)
• Action at a distance (wikipedia - which features a MJD example of unfortunate Perl action at a distance I see! ;-)
• On Coding Standards and Code Reviews (see "Design and Architecture" section, especially "Information Hiding" bullet point)

How can you tell if option A is faster than option B? Benchmark!

I saw old articles² explaining why lexical variables are faster than package variables.

°) or at least: "it used to be so"

²) like: Lexical variables are a bit faster too since Perl doesn’t have to deal with the symbol table

After more than a decade of optimizing the perl interpreter for modern usage (e.g. use strict;), it seems Perl is significantly faster when using "my". Here are my test results:

cavac@earthrise:~/src/temp$ ./speedtest.sh 
Speedtest bash script:
#!/bin/bash

echo "Speedtest bash script:"
cat speedtest.sh
echo 
echo Perl version:
perl -v | grep version
echo
echo "Warming up CPU to operating temp"
cat speedtest_warmup.pl
time perl speedtest_warmup.pl
echo 
time perl speedtest_warmup.pl
echo "########### Modern perl: strict and warnings enabled ###########
+#"
cat speedtest1.pl
echo 
time perl speedtest1.pl
echo "########### Outdated: my variables, but no strict and warnings #
+#######"
cat speedtest2.pl
echo 
time perl speedtest2.pl
echo "########### Noob perl: no strict, warnings or my variables #####
+##"
cat speedtest3.pl
echo 
time perl speedtest3.pl

Perl version:
This is perl 5, version 34, subversion 0 (v5.34.0) built for x86_64-li
+nux

Warming up CPU to operating temp
#!/usr/bin/env perl

# Just to get system load and CPU temp up to test conditions

use strict;
use warnings;

my $total = 0;

for(my $i = 0; $i < 200_000_000; $i++) {
    $total++;
}

real    0m14,558s
user    0m14,543s
sys    0m0,004s


real    0m14,845s
user    0m14,833s
sys    0m0,008s
########### Modern perl: strict and warnings enabled ############
#!/usr/bin/env perl

# STRICT & WARNINGS + MY #

use strict;
use warnings;

my $total = 0;

for(my $i = 0; $i < 300_000_000; $i++) {
    $total++;
}


real    0m22,095s
user    0m22,086s
sys    0m0,004s
########### Outdated: my variables, but no strict and warnings #######
+#
#!/usr/bin/env perl

# MY WITHOUT STRICT AND WARNINGS #

my $total = 0;

for(my $i = 0; $i < 300_000_000; $i++) {
    $total++;
}


real    0m21,943s
user    0m21,938s
sys    0m0,000s
########### Noob perl: no strict, warnings or my variables #######
#!/usr/bin/env perl

# NOOBIE VERSION WITHOUT STRICT; WARNINGS AND MY #

$total = 0;

for($i = 0; $i < 300_000_000; $i++) {
    $total++;
}


real    0m41,117s
user    0m41,100s
sys    0m0,008s
[download]

Does it matter how fast it is if the program doesn't run without it? Always use use strict; use warnings;.