Maybe a 64/32-bit difference? But that works out marginally slower on my machine:

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];

cmpthese -1, {
    one_eval => q[
        my $t = 1;
        eval qq[ sub count { \$_[0] =~ tr[$t][$t] } ];
        my $c = 0;
        $c += count( $_ ) for 1 .. 1e6;
        print $c;
    ],
    many_evals => q[
        my $t = 1;
        my $c = 0;
        $c += eval qq[ \$_ =~ tr[$t][$t] ] for 1 .. 1e6;
        print $c;
    ],
    loop => q[
        my $t = 1;
        my $c = 0;
        for my $n ( 1 .. 1e6 ) {
            ++$c while $n =~ m[$t]g;
        }
        print $c;
    ],
    loop2 => q[
        my $t = 1;
        my $c = 0;
        for my $n ( 1 .. 1e6 ) {
            $c += () = $n =~ m[$t]g;
        }
        print $c;
    ],
};

__END__
C:\test>junk30
600001
600001
600001
            (warning: too few iterations for a reliable count)

600001
600001
600001
            (warning: too few iterations for a reliable count)

600001
600001
            (warning: too few iterations for a reliable count)

600001
Subroutine count redefined at (eval 2000028) line 1.
600001
Subroutine count redefined at (eval 2000029) line 1.
600001
Subroutine count redefined at (eval 2000032) line 1.
600001
            (warning: too few iterations for a reliable count)

                  Rate many_evals      loop2       loop   one_eval
many_evals 3.68e-002/s         --       -97%       -97%       -98%
loop2           1.06/s      2777%         --        -7%       -57%
loop            1.14/s      3006%         8%         --       -53%
one_eval        2.44/s      6515%       130%       113%         --
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Another reason for not using it, is that it gets exponentially slower as the number of hits increases, due to the need to allocate large lists which then get discarded:

perl -MTime::HiRes=time 
-E"$m=1e5;$x='x'x$m; $t=time; $n=()=$x=~m[x]g; say +(time()-$t)/$m"
1.44867897033691e-006

perl -MTime::HiRes=time 
-E"$m=1e6;$x='x'x$m; $t=time; $n=()=$x=~m[x]g; say +(time()-$t)/$m"
1.46089999675751e-005
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