DESCRIPTION
which returns the pathnames of the files (or links) which would be exe�-
cuted in the current environment, had its arguments been given as  com-
mands  in a strictly POSIX-conformant shell.  It does this by searching
the PATH for executable files matching the names of the  arguments.  It
does not canonicalize path names.

##</code><code>##

$ echo $SHELL
/bin/bash

##</code><code>##

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

$ which perl
/usr/bin/perl

$ perl -le 'print $];'
5.026001

$ /usr/bin/perl -le 'print $];'
5.026001

# Select another perl, and hide the
# perl executable by renaming it
$ mv ~/perl528-d/bin/perl ~/perl528-d/bin/perl_hide

# Prepend the location of that
# "hidden" perl to $PATH
$ export PATH=~/perl528-d/bin:$PATH

# That change to $PATH clears the cache,
# so run a perl command that will again
# set the cached perl to /usr/bin/perl.
# The problem does not arise if this 
# step is omitted. 
$ perl -le 'print $];'
5.026001

# Make the hidden perl visible again
$ mv ~/perl528-d/bin/perl_hide ~/perl528-d/bin/perl

$ which perl
/home/sisyphus/perl528-d/bin/perl

$ /home/sisyphus/perl528-d/bin/perl -le 'print $];'
5.028000

$ perl -le 'print $];'
5.026001

##</code><code>##

$ perl -le 'print $];'
5.026001
$ export PATH=~/nonexistent:$PATH
$ perl -le 'print $];'
5.028000