It's not very neat, but you could alter indent() to take an optional second argument which is the maximum amount of leading space to trim. eg:

#!/usr/bin/perl
use strict;

print "Content-type: text/plain\n\n";

print "Test\n\n";

sub indent {
    my ($text, $cols) = @_;
    $cols //= 1000;
    $text =~ s/^\s{0,$cols}//gm;
    return $text;
}

if (1) {
    print indent(<<"END_TEXT", 1);
    Here is
    some test text
    with plenty of space 
        and an indent here
    at the start
END_TEXT
}

exit 0;
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Note that you'll need to be careful with tabs v spaces. The value of 1 for $cols here is fine for a tab. You'll see that with that level, the indent on line 4 of the paragraph is preserved.

PS. indent is probably better named outdent since that is effectively what it is doing.