Please excuse me for not answering your latest question. Based on this new info, my understanding of you real requirement has changed. I now believe that you want to replace all UNIX variable references in a UNIX pathname with their value the same as UNIX does. You make an exception if the variable is not defined. You want to leave the reference in place. (UNIX would replace it with a null string.) This view of the requirements suggests a much different solution. 
use strict;
use warnings;
use Test::More  tests => 5;
my %ENV = (     # Overrides the global for this test
    playground => 'myplay',
    HOME       => 'myhome',
    le1        => 'fix_for_file1'
);
my @expected = (
    q(set playground="myhome/playground")."\n",
    q(set file1=myplay'/fifix_for_file1')."\n",
    q(set file2=myplay'/file2$')."\n",
    q(set file3=myplay'/f$i$l$e$3$')."\n",
    q(set file4=myplay'/fi\$le4')."\n",

);
#                        look
#                        behind  |------ $1 ------|
my $with_braces    = qr/ (?<!\\) ( \$ \{ (\w+) \} ) /x;
my $without_braces = qr/ (?<!\\) ( \$    (\w+)    ) /x;
#                                        |$2 |


while (<DATA>) {
    s!$with_braces   !$ENV{$2}//$1!xge;
    s!$without_braces!$ENV{$2}//$1!xge;
    is( $_ , shift @expected, $_ ); 
}
__DATA__
set playground="${HOME}/playground"
set file1=${playground}'/fi$le1'
set file2=${playground}'/file2$'
set file3=${playground}'/f$i$l$e$3$'
set file4=${playground}'/fi\$le4'

[download]

OUTPUT:

1..5
ok 1 - set playground="myhome/playground"
#
ok 2 - set file1=myplay'/fifix_for_file1'
#
ok 3 - set file2=myplay'/file2$'
#
ok 4 - set file3=myplay'/f$i$l$e$3$'
#
ok 5 - set file4=myplay'/fi\$le4'
#

[download]
Bill

In reply to Re^5: How to replace envs in path? by BillKSmith
in thread How to replace envs in path? by ovedpo15

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