#! c:\perl\bin

print("Enter filename to open (text only):\n\n");
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No

use strict;
use warnings;
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Too bad!

$in=<STDIN>;
chomp $in;
    if ($in !~ /\.txt$/i)
        {
            $in.=".txt";
        }
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chomp(my $in=<STDIN>);
$in .= '.txt' unless $in =~ /\.txt$/i;
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open ($in, "$in") or die("$in could not be opened:$!\n");
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Too bad: not only it features useless quoting of variables, but it's also wrong in that for lexical filehandles to work, the variable must be undefined.

open my $infh, '<', $in or die "$in could not be opened:$!\n";
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        s/\band\b/$word/ig;
                
                while (/\band\b/ig)
                    {
                    ++$count;
                    }
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After you've substituted all occurrances of 'and' with $word, there won't be many remaining unless $word =~ /\band\b/i. Count around the substitution, instead.