Below, benchmarked the same 3 functions with minimal variable declaration.

use warnings;
use strict;

use Benchmark qw{ cmpthese };

# original code by choroba

sub precat {
    my ($string, $prefix) = @_;
    $string = $prefix . $string;
    $string
}
sub Substr {
    my ($string, $prefix) = @_;
    substr $string, 0, 0, $prefix;
    $string
}
sub subst {
    my ($string, $prefix) = @_;
    $string =~ s/^/$prefix/;
    $string
}

# minimal variable declaration

sub precatm {
    $_[1] . $_[0];
}
sub Substrm {
    substr my $string = $_[0], 0, 0, $_[1];
    $string;
}
sub substm {
    ( my $string = $_[0] ) =~ s/^/$_[1]/;
    $string;
}

cmpthese -1, {
    precat  => q( precat  'def', 'abc' ),
    substr  => q( Substr  'def', 'abc' ),
    subst   => q( subst   'def', 'abc' ),

    precatm => q( precatm 'def', 'abc' ),
    substrm => q( Substrm 'def', 'abc' ),
    substm  => q( substm  'def', 'abc' ),
};
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Results from a 2.6 GHz Core i7 CPU:

             Rate   subst  substm  precat  substr substrm precatm
subst   2104367/s      --    -10%    -29%    -48%    -61%    -72%
substm  2338582/s     11%      --    -21%    -43%    -57%    -69%
precat  2969268/s     41%     27%      --    -27%    -45%    -61%
substr  4071762/s     93%     74%     37%      --    -25%    -46%
substrm 5397081/s    156%    131%     82%     33%      --    -29%
precatm 7561845/s    259%    223%    155%     86%     40%      --
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