#!/usr/bin/perl
use warnings;
use strict;

open my $EQ, '<', '1.txt' or die "1: $!";
my %subst;
while (<$EQ>) {
    chomp;                                   # <- updated
    my ($search, $replace) = split /=/;
    $subst{$search} = $replace;
}

my $regex = join '|', map quotemeta, keys %subst;

open my $LST, '<', '2.txt' or die "2: $!";
while (<$LST>) {
    s/($regex)/$subst{$1}/;
    print;
}