And strict complains, because the arguments aren't numeric.

Actually, it's warnings that is complaining.
If you turn stricts off and keep warnings on, you get the message, which is, appropriately enough, a "warning."
When strict complains, it usually means that your program can't execute at all.

#!/usr/bin/perl -w
use strict;

my @x = qw( 1xxx  2xxx 300x  10xx  );

@x = sort @x;
print  "default sort: @x\n";

@x = sort {local $^W; $a <=> $b} @x;
print "numeric sort: @x\n";
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Transforming the comparison elements with int($a) won't keep warnings silent either. Turning off warnings temporarily gets rid of the messages, but I wouldn't recommend it. Warnings are useful because they tell you that something is wrong and needs your attention.

Therefore I would go for the third solution.
However, your regex is not working. It seems to be doing something in your particular program, because the array was already sorted by the previous statement.
Try this.

@x = qw( 5xxx  2xxx 300x  10xx  );
@x = sort {$a =~/^d+/ <=> $b =~/^d+/} @x; # WRONG
print "regex sort: @x\n";
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And your array is not sorted at all.
A better sorting should be

@x = qw( 5xxx  2xxx 300x  10xx  );
@x = sort {my ($y) = $a =~/^(\d+)/; 
           my ($z) = $b =~/^(\d+)/; 
           $y <=> $z} @x;
print "regex sort: @x\n";
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