At least in my environment (AIX), opening the file in append mode without adding any content does not update the modification date of the file.

$: date
Thu Jan  7 09:37:01 CST 2016
$: touch step.file
$: ls -l step.file
-rw-r--r--    1 wlsedi   wlsedi            0 Jan 07 09:37 step.file
$: date
Thu Jan  7 09:38:01 CST 2016
$: touch step.file
$: ls -l step.file
-rw-r--r--    1 wlsedi   wlsedi            0 Jan 07 09:38 step.file
$:  date
Thu Jan  7 09:39:38 CST 2016
$: perl -de 1

Loading DB routines from perl5db.pl version 1.28
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   1
DB<1> open $fh,'>>','step.file'
DB<2> close $fh
DB<3> q
$: ls -l step.file
-rw-r--r--    1 wlsedi   wlsedi            0 Jan 07 09:38 step.file
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Opening in write does have the un-touchlike side effect of wiping out any content. In this particular instance, that is of no consequence, but using the code as a generic replacement for touch is problematic.